i think every ncp should be able to die. i mean in solitude doing dark brotherhood quest killing a person in their brought out argonian. he can't be killed it annoys me he just takes a knee when his health runs out. while the guards die. wish bethesda would make an update adding all npcs can die even the annoying kid in whiterun "i'm not afraid of you". i have a ps3 so modding is out of the question. some characters that are annoying you'd wish you can kill them but for some reason you can't because they're one of the unkillable npc type. i like skyrim just would like for once to kill every person if i choose to in a game including kids. after all its a game not real life. at least make an way to at least kill the advlt unkillable npcs at least that would be a good start if they don't want to add children as a killable npc in their games. i have no idea on how to do it without messing up the main quest if somebody kills a certain npc though but it would make things more interesting. delphine is anothe rcharcter i wanted to kill. trying to do the main quest where i had to go to a party at the thalmor embassy and a dragon appears she rather fight that instead of giving me instructions to continue on until the dragon was dead. delphine is another npc added to the list of charcters i wish could die an dnot take a knee.

..Yep.. this is the only downside for me as well. You can't literally go on a killing spree because of 2 very annoying things(1 of which you have not pointed out).

The high abundance of essential characters
The high abundance of non respawning NPCs.

I think some should turn kill-able in my opinion, such as the jarls who lose their hold.

This really annoyed me too, which is why I made a mod that removes the 'essential' tag from everyone save for a couple of people vital to the MQ and CW quest lines.

Unfortunately, those on console systems are stuck, because Bethesda wimped out rather than make people live with the consequences of killing quest-related individuals, which isn't really fair to those who don't mind such things.

i think they should make it so that everyone can only be killed by you, the player character. Anything else that attacks the current essentials should give them the same effect, the kneeling thing, but when you slice them they should drop dead, with a message saying you failed whatever quest was linked to them. or throw in an awesome questline that leads to a magical weapon that will make essentials die, for console players sake.

That's bugthesda 4u!

You guys might get pissed when the expansion is released and you can't do any added quests because you killed certain people... ever think of that ?

Rather unlikely, since any expansions would come with their own NPCs who would provide quests. True, they could use already-extant NPCs, however it's much easier to make a new one than to untangle a current one from all the quests and dialogue options he or she may be a part of in order to insure that the new ones function properly.

Because then you'd end up with 10 quests where there is an NPC you need to talk to, and you can't, because they are dead.

The ones that really annoy me are the camp leaders. You have helped one side win the civil war, but can't do a proper clean up.
Once again, some perspective, the http://www.uesp.net/wiki/Skyrim:Essential_NPCs of essentials.